The soil and aquatic sediment in urban and industrial areas are prone to contamination resulting from industrial and transportation activities. In general, two major classes of contaminants have been identified, namely "organic" and "inorganic".
The organic contaminants, designated as total petroleum hydrocarbons (TPH) are usually water insoluble and are often adsorbed on the surface of mineral grains comprising soil or aquatic sediment. The inorganic contaminants, which comprise for example arsenic (As), barium (Ba), cadmium (Cd), chromium (Cr), copper (Cu), cobalt (Co), gold (Au), lead (Pb), mercury (Hg), molybdenum (Mo), nickel (Ni), selenium (Se), silver (Ag), tin (Sn), zinc (Zn), are found in the sediment matrix and in the soil as minerals, metal, chemical complexes or alloys. The concentrations of these specific elements with the exception of gold and silver are considered pollutants and are presently regulated by environmental agencies.
According to literature data (review in: U.S. EPA, 1994), "organic" and "inorganic" contaminants are mainly present in the soil and sediment fractions inferior to 45 microns in average particle size. This fraction often constitutes, on a dry basis, the bulk of the soil and sediment.
In harbour areas for instance, continuous wharf access for ships requires regular dredging of the harbour sediments. Because of pollution problems, sediment disposal associated with dredging operations is now regulated. Direct water disposal of contaminated dredged sediments is prohibited. Land-based disposal sites are still available, however the contamination level and the volume of sediments often renders these confinement operations very costly.
Current sediment decontamination techniques are summarized by Averett and Francingues, Sediment Remediation: An International Review; Second International Conference on Dredging and Dredged Material Placement--Volume 1;13-16 November 1994; Walt Disney World, Lake Buena Vista, Fla.; New York: American Society of Civil Engineers. Current chemical, thermal and biological treatment methods are plagued by their high costs and their difficulties in simultaneously handling organic and inorganic contaminants.
In Germany, Detzner, H. D.; Kitschen L., and Weimerskirch W., METHA--The first large-scale plant for treatment of harbour sediments; Aufbereitungs-Technik Mineral Processing. 1993; Vol. 34(No. 5):235-242 report a sediment remediation method using particle size separation. The technology uses hydrocyclones separation for sediment volume reduction. The sediment fraction smaller than 45 microns is not treated.
In the Netherlands, Van Rijt, C. Cleaning contaminated sediments by separation on the basis of particle size; Water Science and Technology 1994; Vol. 28 (No. 8-9):283-295. reports a similar approach at pilot scale. Again the sediment fraction smaller than 45 microns is not treated.
In the USA, Traver, R. P., 1996, Conditioning sediment and soil for site remediation; "Cleaning contaminated sediment Symposium", College of Engineering, University of Wisconsin, Madison, Wis., 19 p., Sep. 19, 1996 describes a pilot-scale sediment washing system evaluated for sediments contaminated with heavy metals. The heavy metals contained in the sediment fraction consisting of material larger than 45 microns are concentrated using flotation cells. The fine material, smaller than 45 microns is separated from the bulk of the sediment by hydrocycloning and are directly disposed of (without any treatment) at a specialized landfill at high costs. In fact one of the requirements, which greatly limit the usefulness of this technology to the treatment of fine sediment, is that "70% of the material to be treated must be greater than 45 microns in size" (Traver, 1996, p. 3).
U.S. Pat. No. 4,851,123 teaches the treatment of oily sludge by separating hydrophobic and hydrophilic components. The separation consists of three steps. The first step is based on size, the second on density, and the third on physicochemical differences. It is at the third step that the smaller and lighter fraction is routed to a flotation cell for the removal of the hydrophobic component.
EP 647, 483 teaches a similar method as U.S. Pat. No. 4,851,123, wherein a flotation cell is used to float out the hydrophobic component.
The United States Environmental protection agency (EPA) has conducted preliminary studies on hydrocarbon removal of aquatic sediments by froth flotation using flotation cells. (Assessment and Remediation Of Contaminated Sediments (ARCS) Program--Mineral Processing Pretreatment of Contaminated Sediments. Chicago, Ill.: Great Lakes National Program Office; 1994 October; EPA 905-R94-022). The EPA results indicated poor efficiency for material smaller than 45 microns. The EPA concluded that the technologies they had tested were not appropriate for the treatment of contaminated fine particles.
This agency also conducted reviews of current soil decontamination techniques (EPA, Engineering Bulletin--soil washing treatment Chicago, Ill.: Great Lakes National Program Office, (1990) EPA 540/2-R90-017 and EPA, Technology
Profiles Eight Edition (1995) SITE ). The EPA concluded that the technologies they had tested were not appropriate for the treatment of contaminated fine particles of soil.
Thus there remains a need to develop an effective treatment method for sediment and soil remediation for the decontamination of material 45 microns or less.
It is an object of the present invention to provide a very fine (45 microns or less) particle treatment method characterized by cost-efficient handing of large volumes of sediment.
It is a further object of the present invention to provide a method wherein the decontamination performance meets all applicable environmental standards for returning decontaminated sediment to its aquatic environment or to non-specific landfill sites.
It is a further object of the present invention to provide a method wherein the decontamination performance meets all applicable environmental standards for returning the decontaminated soil to non-specific landfill sites.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that this detailed description, while indicating preferred embodiments of the invention, is given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art.